Walking along the Friedrich-Adolf-Richter-Straße in Rudolstadt, Germany, visitors may not notice anything unusual about the bright yellow multi-family house that occupies a large chunk of the street. But fewer than 20 years ago, the building was a sullied, run-down Soviet military barracks, vacated as the Red Army withdrew from the former East Germany.

Welcome to the new face of sustainable building, where dirty old factories and power plants, even military facilities, are rescued and turned into sustainable homes, shops and offices.

“Affordability and location are the most important aspect when people look for housing, and then comes sustainability”, said Arndt Hobrecker, director of properties at LEG Thüringen, the agency that remodels and rents out former barracks in the central German state of Thuringia, where Rudolstadt is located. “And the substance of these buildings, with thick walls that conserve heat in winter and cold in summer, makes them sustainable in themselves.”

With some 800 military barracks, airports and other military facilities left vacated by the Red Army, cities in the former East Germany are a virtual laboratory for how sullied but solid old buildings can be reused in an eminently sustainable fashion.

“Adapting and reusing such buildings is a very good step from an environmental perspective”, explained Rajat Gupta, professor of sustainable architecture and climate change and director of the Oxford Institute for Sustainable Development (OISD). “And unlike modern buildings, they’re usually heavy-weight. As we move to a warmer climate, it’s better to live in such buildings because they keep the heat out.”

Factories have, of course, been re-used for decades, but the sustainability focus is new. So is the availability of buildings in cities across Europe and the US as armed forces downsize and manufacturing moves to Asia.

In Montreal, a former train car factory has been turned into LEED-certified housing and offices. In New York, the National Audubon Society is now housed in a former garment factory, remodeled and using 62% less energy than regular office buildings. Chicago has even seen a former food factory turned into a vertical farm.

“In the goal for sustainable design, reusing the embodied energy in an existing building is helpful”, said Michael Garrison, professor of architecture at the University of Texas, Austin. “Most building materials can be reused and many buildings can be remodeled, extending the life of the material or the building far into the future and saving significant amounts of embodied energy.”

According to a 2012 study, retrofitting an old building to make it 30% more energy-efficient is greener than building a new one with the same energy use. In other words: saving factories makes sustainability and business sense.

In Philadelphia, a former manufacturing capital that no longer needs its factories, residents can now choose to live in the Rag Flats, a former rag factory, or the Capital Flats, a former meat-packing plant. Both have been turned into modern apartment buildings featuring roof gardens, solar panels, and water collection. “This city used to be an industrial hub and no longer is, but people are moving back into the city”, explained Timothy McDonald, president of Onion Flats, the firm behind the Rag Flats and Capital Flats conversion. “These kinds of buildings aren’t built anymore. And look at the effects of global warming. Saving buildings like these is just common sense.”

But there’s a problem: toxins. Thuringia’s Red Army barracks have had to be painstakingly stripped of lead pipes, toxic paint and petrol in the ground has had to be removed. Like other Warsaw Pact states, East Germany had the now-good fortune of only being able to afford small amounts of chemicals and asbestos.